1. Field of the Invention
The present invention relates to a hole drilling method and a machine tool.
2. Description of the Related Art
In the prior art, a machine tool which rotates a drill or other rotary tool to drill a hole in a workpiece is known. When drilling a hole in a workpiece, it is possible to rotate the drill or other rotary tool about its axis while moving the rotary tool toward the workpiece so as to form the desired hole. In such hole drilling, it is preferable to drill a hole of a desired shape at a desired position. That is, the machining accuracy of the hole is preferably high.
In this regard, when drilling a hole, the dimensional accuracy sometimes falls at the inside of the hole or the machining accuracy of the hole otherwise deteriorates. In particular, for drilling a deep hole where the depth is greater than the diameter of the hole, it is necessary to use a drill with a small diameter and a long axial direction. For this reason, sometimes the drill becomes lower in rigidity and the hole becomes lower in machining accuracy.
Japanese Patent Publication No. 61-4608A discloses a numerically controlled lathe which uses a drill mounted on a mill shaft to drill a hole in a workpiece which is secured by a chuck. In this lathe, when a flag which instructs drilling of a deep, small diameter hole is stored in the memory region, the workpiece and the drill are made to rotate in opposite directions. That is, it is disclosed to rotate the workpiece and rotate the drill and to make the drill rotate in the opposite direction to the rotating direction of the workpiece.
When drilling a hole, the drill etc. is rotated while gradually inserting it into the workpiece. The machining accuracy of the hole includes the dimensional accuracy of the change in shape of the hole in the depth direction and the straightness accuracy of the hole (or bending accuracy of the hole). In drilling a hole, it is preferable to drill it while suppressing deterioration of the machining accuracy.
As disclosed in the above Japanese Patent Publication No. 61-4608A, it is possible to improve the machining accuracy by making the workpiece and drill rotate in opposite directions to each other on the same axis while making them approach. In particular, in the drilling of a deep hole, it is possible to drill a hole by a high machining accuracy.
In this regard, in this drilling, it is necessary to make the workpiece rotate so that the depth direction of the hole is aligned with the axis of rotation of the workpiece. It is necessary to arrange the position of drilling the hole on the axis of rotation of the workpiece. For this reason, when drilling a plurality of holes in a single workpiece, it is necessary to change the position of the workpiece each time drilling a hole. That is, when drilling a plurality of holes, it is necessary to refasten the workpiece to the workpiece rotating device so that the position of the hole is aligned with the axis of rotation of the workpiece. There is therefore the problem that the work takes time and trouble.
Further, since fastening the workpiece so that the position of the hole is aligned with the axis of rotation of the workpiece, sometimes unbalance occurs when making the workpiece rotate or the end of the workpiece strikes the machine tool making machining impossible. For example, when drilling a hole at one end of a long, thin workpiece, it is necessary to arrange that end on the center axis of the workpiece rotating device. In this case, sometimes the other end at the opposite side to that one end sticks out from the rotating device. As a result, when rotating the workpiece, sometimes the distance between the center of gravity position of the workpiece and the axis of rotation ends up becoming large and unbalance is caused. Alternatively, sometimes the other end strikes the machine tool. As a result, there is the problem that the workpiece which can be drilled with a hole becomes smaller.